std::basic_string<CharT,Traits,Allocator>::substr

 

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std::basic_string

Member functions basic_string::basic_string basic_string::~basic_string basic_string::operator= basic_string::assign basic_string::assign_range (C++23) basic_string::get_allocator Element access basic_string::at basic_string::operator[] basic_string::front (DR*) basic_string::back (DR*) basic_string::data basic_string::c_str basic_string::operator  basic_string_view (C++17) Iterators basic_string::beginbasic_string::cbegin (C++11) basic_string::endbasic_string::cend (C++11) basic_string::rbeginbasic_string::crbegin (C++11) basic_string::rendbasic_string::crend (C++11) Capacity basic_string::empty basic_string::sizebasic_string::length basic_string::max_size basic_string::reserve basic_string::capacity basic_string::shrink_to_fit (DR*) Modifiers basic_string::clear basic_string::insert basic_string::insert_range (C++23) basic_string::erase basic_string::push_back basic_string::pop_back (DR*) basic_string::append basic_string::append_range (C++23) basic_string::operator+= basic_string::replace basic_string::replace_with_range (C++23) basic_string::copy basic_string::resize basic_string::resize_and_overwrite (C++23)   basic_string::swap

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	(1)
basic_string substr( size_type pos = 0, size_type count = npos ) const;			(until C++20)
constexpr basic_string substr( size_type pos = 0, size_type count = npos ) const;			(since C++20) (until C++23)
constexpr basic_string substr( size_type pos = 0, size_type count = npos ) const&;			(since C++23)
constexpr basic_string substr( size_type pos = 0, size_type count = npos ) &&;		(2)	(since C++23)

Returns a substring [pos, pos + count). If the requested substring extends past the end of the string, i.e. the count is greater than size() - pos (e.g. if count == npos), the returned substring is [pos, size()).

Parameters

pos	-	position of the first character to include
count	-	length of the substring

Return value

String containing the substring [pos, pos + count) or [pos, size()).

Exceptions

std::out_of_range if pos > size()

Complexity

Linear in count

Notes

The returned string is constructed as if by basic_string(data() + pos, count), which implies that the returned string's allocator will be default-constructed — the new allocator might not be a copy of this->get_allocator().

For the overload (2) (with && ref-qualifier), *this is left in a valid but unspecified state.

(since C++23)

Example

Run this code

#include <iostream>
#include <string>
 
int main()
{
    std::string a = "0123456789abcdefghij";
 
    // count is npos, returns [pos, size())
    std::string sub1 = a.substr(10);
    std::cout << sub1 << '\n';
 
    // both pos and pos+count are within bounds, returns [pos, pos+count)
    std::string sub2 = a.substr(5, 3);
    std::cout << sub2 << '\n';
 
    // pos is within bounds, pos+count is not, returns [pos, size())
    std::string sub4 = a.substr(a.size() - 3, 50);
    // this is effectively equivalent to
    // std::string sub4 = a.substr(17, 3);
    // since a.size() == 20, pos == a.size()-3 == 17, and a.size()-pos == 3
 
    std::cout << sub4 << '\n';
 
    try
    {
        // pos is out of bounds, throws
        std::string sub5 = a.substr(a.size()+3, 50);
        std::cout << sub5 << '\n';
    }
    catch(const std::out_of_range& e)
    {
        std::cout << "pos exceeds string size\n";
    }
}

Output:

abcdefghij
567
hij
pos exceeds string size

See also

copy	copies characters (public member function)
sizelength	returns the number of characters (public member function)
find	finds the first occurrence of the given substring (public member function)
constexpr size_type npos [static]	the special value size_type(-1), its exact meaning depends on the context